How to share data resources between widgets in GWT

I am using GWT and AppEngine for a project. I would like to know how can I share data (ArrayList objects)between widgets, so I could centralize the logic and reduce the number of RPC calls to the server.

I have thought of two ways, but I don't know which is better:

1) When I instantiate the widget, I pass the ArrayList object as a parameter, although I don't know how to do that because the widget gets instantiated with :

  ThisAppShell shell = GWT.create(ThisAppShell.class);    

2) By using a mechanism like eventBus

http://www.dev-articles.com/article/Gwt-EventBus-(HandlerManager)-the-easy-way-396001

When the user loads the application,after the login process is complete, I would like to download a list of employees which should be available for all widgets. This should all be done in the onModuleLoad() method. I would like to download them all at startup because I would like to implement some sort of caching mechanism. For example, I want to have 2 ArrayList instances: - emplListOnStart which is populated when the application is loading - emplListChanges, an array on which the user will make modifications from inside widgets.

After the user has finished making the changes (he presses the "Save" button), the two arrays will be compared, the differences will be saved in appengine (via RPC) and also updated in emplListOnStart.

This is the code for the EntryPoint class:

public class ThisApp implements EntryPoint {
ThisAppShell shell = GWT.create(ThisAppShell.class);
LoginServiceAsync loginService = GWT.create(LoginService.class);

private ArrayList<Employee> emplListOnStart;

private ArrayList<Employee> emplListChanges;

public void onModuleLoad() {
    RootLayoutPanel.get().clear();
    RootLayoutPanel.get().add(shell);
    loginService.isAuthenticated(new AsyncCallback<UserDto>() {

        public void onFailure(Throwable caught) {
            // TODO Auto-generated method stub

        }

        public void onSuccess(UserDto result) {
                             //Here I should load the emplListOnStart list;
        }

    });
    shell.getLogoutLink().addClickHandler(new ClickHandler() {
        public void onClick(ClickEvent event) {
            loginService.logout(new AsyncCallback() {
                public void onFailure(Throwable caught) {

                }

                public void onSuccess(Object result) {
                                             //Here the user will get logged out
                }
            });
            Window.Location.assign("");
        }
    });

   }
}

And here is the code for the widget:

public class ThisAppShell extends Composite {

private static ThisAppShellUiBinder uiBinder = GWT
        .create(ThisAppShellUiBinder.class);

interface ThisAppShellUiBinder extends UiBinder<Widget, ThisAppShell> {
}

@UiField
Anchor logout_link;
@UiField
StackLayoutPanel stackLPanel;
@UiField
TabLayoutPanel tabLPanel;

public ThisAppShell() {
    initWidget(uiBinder.createAndBindUi(this));

    initializeWidget();
}

public void initializeWidget() {
    stackLPanel.add(new HTML("Manage empl."), new HTML("Employees"), 30);
    stackLPanel.add(new HTML("Manage Dept."), new HTML("Departments"), 30);

    // Add a home tab
    HTML homeText = new HTML("This is the home tab");
    tabLPanel.add(homeText, "Home");

    // Add a tab
    HTML moreInfo = new HTML("This is the more info tab");
    tabLPanel.add(moreInfo, "More info");

    // Return the content
    tabLPanel.selectTab(0);
}

public Anchor getLogoutLink() {
    return logout_link;
}

}

Is this possible, or how could this be done better?

Thank you.

I think there are two ways to do it:

1. Create a setter on your widget to set your ArrayList instances (setData()). You can then call this function in the onSuccess method of your loginService.

2. Inject the singleton instance of a global EventBus into your widget (using i.e. gin/guice) and fire an event containing your data. In the widget you have to attach an EventHandler for the specific event (i.e. LoadEmplListEvent).

I think both solutions are fine to use. Solution one creates a tighter coupling to your widget but is easier to implement and I think you should take this route if you only have a small number of widgets where you work with the data.

Solution is a cleaner approach because it de-couples your widgets from the rest. You fire the event the data in your onSuccess method once and you don't care about the widgets.
The widgets that are interested in the data will make sure that they handle the event appropriately (by handling the event). I guess if you have a lot of widgets that have to deal with the data the second approach is the one to go for.